1. Field of the Invention
The present invention relates to a slurry recycling method. More specifically, the present invention relates to a slurry recycling method for recovering a dispersion medium and abrasive grains for recycle use from a spent slurry which is generated when brittle materials such as, for example, polycrystalline silicon for solar battery, semiconductor materials, magnetic materials and ceramics are sliced by means of a multi-wire saw (hereinafter, abbreviated as “MWS”) using a slurry in which abrasive grains are dispersed in a dispersion medium.
2. Description of the Background Art
Generally, a conventional method for recovering a dispersion medium and abrasive grains from a spent slurry separates and recovers a dispersion medium and abrasive grains by using a centrifugal separator as is disclosed in Japanese Unexamined Patent Publication HEI 11(1999)-156719. In this conventional art, a recycled slurry is produced from a spent slurry in the following manner.
First a spent slurry is centrifuged by a centrifugal separator (primary) at super low G in the range of 200 to 1200 G (this process is generally called as “primary separation”) to be separated into a high density liquid mainly containing abrasive grains and a low density liquid mainly containing silicon dust. The high density liquid mainly containing abrasive grains is generally called as an abrasive grain recovery liquid or a primary recovery liquid. The low density liquid mainly containing silicon dust is then applied to a centrifugal separator (secondary) at 2000 to 3500 G (this process is generally called as “secondary separation”) to be separated into a solid (commonly called as “sludge”) composed of dust and abrasive grains (not having recovered in the primary separation or having grained) and a dispersion medium. Thereafter, the high density liquid mainly containing abrasive grains obtained in the first centrifugation and the dispersion medium obtained in the second centrifugation are mixed with each other, and additionally fresh abrasive grains and a fresh dispersion medium are mixed thereto based on the specific gravity and viscosity, to produce a recycled slurry. This recycled slurry can be used again for the MWS.
According to the method as described above, when the amount of silicon dust in the spent slurry is 5% by weight or less, a slurry for slicing wafers can be produced again accompanied with little medium to be disposed. However, when the amount of silicon dust in the spent slurry is 5% by weight or more, the cutting performance of abrasive grains is impaired so that defects such as unevenness in thickness (TTV), warping and breakage frequently occur in wafers after slicing to deteriorate the yield. Additionally, not only breaking occurs in the slicing wire to render the yield 0%, but also the main unit of the multi-wire saw also suffers critical damage (for example, breakage of wire guide) to lead deterioration of availability.
Furthermore, in the case where a certain amount of slurry is used with being pooled in a tank until slicing of wafers completes using a water-soluble or aqueous dispersion medium, or a small amount of slurry is used with being circulated, the amount of silicon dust may become 12% by weight or more during slicing. In such a case, the viscosity of the slurry increases. Due to the increased viscosity the slurry may reside between wafers so that the wafers spread in skirt forms (peach forms), to hinder drawing wire. Or if the drawing is possible, the wire breaks the wafers so that the yield may deteriorate. Also solid substances may adhere on the surface of the wafers and further labor and time may be required for cleaning the adhered substances.
The similar problems occur also in the case where an oily dispersion medium is used and 15% by weight or more of silicon dust is contained in a slurry during slicing.
In order to prevent such problems, from several % to as large as about 70% of recycled dispersion medium obtained in the secondary separation is disposed.
Additionally, in the case where the slurry is not recycled as described above, a certain amount of or the entire spent slurry is drawn out from the tank after completion of slicing and disposed. Then a dispersion composed of fresh abrasive grains and a fresh dispersion medium is mixed in the tank for use. The amounts of the fresh abrasive grains and the fresh dispersion medium are determined while checking the degree of occurrences of warping, TTV, breakings of wafers, as well as the accuracy as is in the case where a slurry recycling apparatus is used. In general, a half or one-third of tank capacity of the spent slurry is disposed, and the corresponding amount of dispersion wherein fresh abrasive grains and a fresh dispersion medium are mixed is mixed thereto so as to produce a new slurry.
Regardless of recycling or not recycling the slurry, the expensive abrasive grains and dispersion medium need to be disposed. Furthermore, since such waste is disposed as industrial waste, the cost for such waste also increases the value of wafers obtainable by slicing.